Day surgery under general anesthesia is now common and continues to grow in popularity owing to patient convenience and medico-economic pressures. Perioperative care improvements have allowed surgeons to perform more invasive day-surgical procedures. Additionally, the availability of drugs, such as propofol and remifentanil, has improved anesthesia care for more extensive surgeries. After surgery, especially day surgery, patients usually desire discharge as soon as possible.
Saccadic eye movements can be used to monitor recovery from general anesthesia. Indeed, evaluation of saccadic eye movements is more sensitive than choice-reaction tests in detecting the residual effects of anesthesia. Also, evaluation of saccadic eye movements is more reliable than subjective state-of-alertness tests, such as the visual analogue score for sedation, owing to the tendency for subjects to underestimate their impairment.
Eye movements may be affected by alcohol or other drugs. The effects of alcohol on saccadic eye movements were reported over three decades ago. Indeed, in field sobriety tests, law enforcement officers are trained to recognize end-point nystagmus that can be elicited by having the subject gaze laterally to the extreme. At least one study has described the change in saccadic eye movements as a measure of drug induced central nervous system (CNS) depression caused by Valium (diazepam). In the mid-1980s, the effects of barbiturates, benzodiazepines, opiates, carbamazepine, amphetamine and ethanol on saccadic eye movements were observed using a computer system coupled to a television monitor that provided visual stimulation for the subject, and an electrooculogram that measured eye movements. Not surprisingly, barbiturates, benzodiazepines, opiates, carbamazepine, and ethanol reduced peak saccadic velocity while amphetamine increased it. Saccadic eye movements have been studied in subjects who were given nitrous oxide or isoflurane. No significant differences were found between air and nitrous oxide. However, isoflurane caused significant diminution of mean saccadic peak velocity. In contrast, there was little effect caused by nitrous oxide or isoflurane on subjective assessment, assessed by subject's reporting of odor, tiredness, drowsiness, sleepiness, or nausea. It has also been reported that both cyclopropane and halothane depressed peak velocity of saccadic eye movements in a dose-dependent fashion. Peak saccadic velocity returned to baseline within 5 minutes after discontinuation. As found with isoflurane, no significant difference was found between halothane, cyclopropane, and air in subjective assessment of impairment. In a separate placebo-controlled trial, a diminution was found in peak saccadic velocity after propofol infusion. A study of the effect of isoflurane on some psychometric measurements showed that isoflurane diminished peak saccadic velocity, increased choice reaction time, and decreased visual analogue scores for sedation, but did not change the critical threshold for flicker fusion. It has been suggested that a combination of peak saccadic velocity, percentage error and choice reaction time would be a potentially useful battery of tests to assess recovery from anesthesia.
More recently, the effect of isoflurane has been studied regarding (1) saccadic latency and (2) a countermanding task. In a saccadic latency test, a moving target comprising a light-emitting diode was displayed on a screen. The latency of eye movements after target movements was measured, and was found to increase with anesthetic dose. In the countermanding task, which requires a higher level of conscious performance, the subject was asked to voluntarily suppress gaze movement to the target. Again, anesthetic increased the latency of response. Both tasks were equally impaired at sub-anesthetic concentrations of isoflurane.
Emergence from anesthesia and return of cognitive function is faster using a combination of propofol and remifentanil as compared to desflurane and sevoflurane. Hence, the propofol-remifentanil combination has become increasingly popular among anesthesiologists.
Measuring saccadic eye movements is a reliable and sensitive method to assess residual effect general anesthesia. Existing methods of measuring saccadic eye movement include electro-oculography (EOG) and use of high-speed video.
EOG has long-been available, and is probably the most widely used method for measuring horizontal eye movement in the clinic setting. EOG is a technique that can record a wide range of horizontal eye movements (±40°) but is less reliable for vertical eye movements. EOG uses the fact that a normal eyeball globe is an electrostatic dipole. The cornea is 0.4-1.0 mV positive relative to the opposite pole. Cutaneous electrodes are placed on both sides of the orbit. The potential difference recorded depends on the angle of the globe within the orbit.
Video detection of saccadic eye movements has been used. Normal video frame rates, however, of 30 Hz are slow relative to the high-speed saccade. Eye tracking devices, however, do exist for tracking this high-speed event using video speeds of 240 Hz or more. Currently the highest available is 480 Hz. These devices are typically complicated and delicate, range in price from $10,000 to $40,000, and often use high-speed cameras, precise delicate optics, image processors, image analysis software, and timed illumination sources to measure saccades.